The "Compatible Time-Sharing System" was an operating system written by a
team of MIT programmers led by Prof. Fernando J. Corbato.

CTSS originally was presented in a paper at the 1962 Spring Joint Computer Conference,
even though the software wasn't quite working. By the time of the summer study it worked.

The hardware that made CTSS run added several instructions and a memory boundary
register to the processor: and two 32K core memory banks, instead of one. Core bank 'A'
held the CTSS supervisor, and 'B' was used for user programs. Many instructions were
forbidden: I/O instructions; attempts to switch core banks; executing them caused the
machine to trap.

CTSS was "compatible" in the sense that FMS could be run in B-core as a
"background" user, nearly as efficiently as on a bare machine. Background could
access some tape units and had a full 32K core image. This feature allowed the computer
users to make the transition from batch to timesharing gradually, and to retain the
ability to run "dusty decks" and software from other institutions.

The plan was to use CTSS to support Multics development, and the last big CTSS project
was a new, much better, file system, to ensure that the system would be adequate to
support Multics development.